Re: PROBLEM: DoS Attack on Fragment Cache

[Eric Dumazet]

On Sat, Apr 17, 2021 at 2:31 AM David Ahern <dsahern@xxxxxxxxx> wrote:
>
> [ cc author of 648700f76b03b7e8149d13cc2bdb3355035258a9 ]



I think this has been discussed already. There is no strategy that
makes IP reassembly units immune to DDOS attacks.

We added rb-tree and sysctls to let admins choose to use GB of RAM if
they really care.



>
> On 4/16/21 3:58 PM, Keyu Man wrote:
> > Hi,
> >
> >
> >
> >     My name is Keyu Man. We are a group of researchers from University
> > of California, Riverside. Zhiyun Qian is my advisor. We found the code
> > in processing IPv4/IPv6 fragments will potentially lead to DoS Attacks.
> > Specifically, after the latest kernel receives an IPv4 fragment, it will
> > try to fit it into a queue by calling function
> >
> >
> >
> >     struct inet_frag_queue *inet_frag_find(struct fqdir *fqdir, void
> > *key) in net/ipv4/inet_fragment.c.
> >
> >
> >
> >     However, this function will first check if the existing fragment
> > memory exceeds the fqdir->high_thresh. If it exceeds, then drop the
> > fragment regardless whether it belongs to a new queue or an existing queue.
> >
> >     Chances are that an attacker can fill the cache with fragments that
> > will never be assembled (i.e., only sends the first fragment with new
> > IPIDs every time) to exceed the threshold so that all future incoming
> > fragmented IPv4 traffic would be blocked and dropped. Since there is no
> > GC mechanism, the victim host has to wait for 30s when the fragments are
> > expired to continue receive incoming fragments normally.
> >
> >     In practice, given the 4MB fragment cache, the attacker only needs
> > to send 1766 fragments to exhaust the cache and DoS the victim for 30s,
> > whose cost is pretty low. Besides, IPv6 would also be affected since the
> > issue resides in inet part.
> >
> > This issue is introduced in commit
> > 648700f76b03b7e8149d13cc2bdb3355035258a9 (inet: frags: use rhashtables
> > for reassembly units) which removes fqdir->low_thresh, and GC worker as
> > well. We would gently request to bring GC worker back to the kernel to
> > prevent the DoS attacks.
> >
> > Looking forward to hear from you
> >
> >
> >
> >     Thanks,
> >
> > Keyu Man
> >
>